Annual payment wud be mistake because BB plans rates keep decreasing incase of BSNL/MTNL.
They can adjust the annual payment, not an issue. But yes, the savings with upfront annual payment are not great when you consider that you do not get any interest and could have used money elsewhere. It is a small saving, so go for it only if you have the extra cash, and are sure you will not want to migrate from BSNL to another ISP.